ox sensor on sequential efi,high overlap cam....

[Jemison Richard]

I thought I'd post an exerpt from the Accelerated Motion web page  -
http://www.amotion.com/amcams.html#500
Hope it helps

Rick

Lobe Center Separation is as big a consideration as duration. This is what
determines how much valve overlap (the amount of the intake and exhaust
valves are open at the same time) your engine will have and what your vacuum
and idle quality will be. Street cams with wide lobe center separation (114)
will have a good idle, high vacuum, and a nice wide RPM range. Separations
closer to 108 (less separation means more valve overlap) will create
problems for some computer engine controls due to their rougher idle and
lower manifold vacuum. They have a shorter RPM range but produce much
stronger mid-range power with some improvement to the top end. 
Now we get to valve lift. Many customers believe that the cam with the
highest lift will perform much better than a moderate lift cam. While it is
true that a high lift cam will provide better flow by getting the valve
further out of the way, there are limits to this as well as other
considerations. 
First, high lift on a short duration cam would be very hard on the valve
train if it could keep the valve at the higher lift long enough to be
effective. 
Second, high lift causes several problems in the valve train including valve
to piston interference (especially with narrow lobe separations), valve
spring retainer to valve guide/seal interference, rocker arm to stud
interference, and valve spring coil bind. Adjustable rocker arms are often
required to take up any clearance created when the valve is closed. All of
this must be checked and corrected before a high lift cam can operate
properly. 
Finally, if you are using a cam with enough duration to make a high lift
effective, the usual limit for noticable improvement is reached when the
lift equals 25% of the intake valve diameter. This means that if your intake
valve diameter is only 1.84" you will get the best flow at only .460" lift.
Lifts higher than 25% of the valve diameter will add more duration at the
maximum flow point but excessive lifts will cause more problems than they
are worth in an engine that is not fully race prepared. 
The bottom line on cam lift is that it is pointless to go overboard. All
newer designs have adequate lift for the operating range of the cam. The
minimal gains from an extra high lift cam are not worth the extra work in a
street vehicle. 


> -----Original Message-----
> From:	Jemison Richard [SMTP:JemisonR at tce.com]
> Sent:	Wednesday, December 02, 1998 3:51 PM
> To:	'diy_efi at efi332.eng.ohio-state.edu'
> Subject:	RE: ox sensor on sequential efi,high overlap cam....
> 
> Tom,
> 
> Sorry for the confusion.  We're on the same page.
> 
> > -----Original Message-----
> > From:	Tom Sharpe [SMTP:twsharpe at mtco.com]
> > Sent:	Wednesday, December 02, 1998 1:48 PM
> > To:	diy_efi at efi332.eng.ohio-state.edu
> > Subject:	Re: ox sensor on sequential efi,high overlap cam....
> > 
> > 
> > 
> > Jemison Richard wrote:
> > <snip>
> > 
> > >         [Jemison Richard]  Turbos actually do care I believe.  Most
> > turbo
> > > cams are wide center line, mild duration, moderate to high lift cams.
> > You
> > > don't want to lose all that pressure out the exhaust pipe!  Also,
> > cylinder
> > > filling is largely taken care of my the pressure developed by the
> turbo.
> > So
> > > you don't need cylinder scavenging.  You already have a large pressure
> > > differential across the cylinder to remove combustion gases and
> recharge
> > the
> > > cylinder.
> > >
> > >          Also,  grab your favorite cam grinders spec sheets or catalog
> > and
> > > compare hot street and turbo cams and check me out on this.
> > >
> > >         Rick
> > 
> > Two points, Removing the combustion gasses is a problem with Turbo
> motors
> > as the
> > exhaust pressure can be higher than the intake. They take a standard,
> mild
> > cam
> > and crutch the exhaust by openning it sooner; more time to blow down.
> Net
> > result
> > is wider CL.
> 	[Jemison Richard]  Got a megacycle cams sheet in front of me for 750
> Honda (sorry this is the only one I have right now but for reference they
> list a high perf street cam with I/E order  Lift  .400 / .375   dur at .04
> 262/257  Centerline  105.104.5
> 	their turbo cam for same bike
> Lift  .342 / .312   dur at .04  215/222  Centerline   106/106
> 
> 	This is fairly typical.  That was my point.  Less lift - I agree
> earlier exhaust opening but not a whole lot -  very conservative durations
> and wider centerline (less overlap).
> 
> > You're right though, boost is better than lotts of cam and RPM, but a
> > turbo at
> > high rpm will still need a big cam thou not as big as a na motor.
> > 
> > My point was (originally), that we should tune the idle by driveability
> > and
> > 'dead recon' the idle mixture from there and forget all that learning
> > stuff (at
> > idle). Just use the O2 for a reference. That assumes that we can easily
> > change
> > the map and have a good trimmer for the map sensor.
> > 
> > My 482 idled like a kitten with a Predator carb, alcohol, and roller
> > cam..... it
> > just dribbled raw fuel out of the headers at idle, not enough to clean
> the
> > floor, but almost. (another sick try at humor).
> 	[Jemison Richard]  Yea, the roller cam does wonders for idle
> quality.  I never had problems with the fuel literally running down the
> pipes but the low end mixture was major goofy. 
> 
> 
> > later  Tom
> >